Beyond the Coastline: The Role of Offshore Data in Understanding Coastal Aquifers

 Traditionally coastal aquifers have been monitored and modelled by restricting the spatial extent of observations and simulations to the inland portion of the aquifer. As a result, most hydrogeological studies and numerical models focus on the terrestrial part of the system, assuming that the key marine processes affecting hydrogeological dynamics are adequately captured at the coastline interface. However, important hydrological processes take place offshore, and may significantly influence the inland aquifer behaviour.

Offshore processes such as tides, sea storms, and even groundwater discharge can generate rapid variations in pressure and salinity in the submerged part of coastal aquifers. These processes operate at temporal scales typically shorter than those governing inland groundwater flow, which is mainly controlled by seasonal fluctuations. Consequently, aquifer dynamics and behaviour may not be fully captured when observations are limited to the terrestrial domain.

In this contribution, we show how offshore monitoring data, such as salinity measurements or geophysical observations can improve the understanding of inland aquifer behaviour. Offshore data provide direct information on marine-driven dynamics that cannot be inferred from inland observations alone and help to better constrain the conceptualisation of coastal aquifer systems.

We further demonstrate that integrating offshore observations into hydrogeological numerical models improves their representativity and ability to reproduce observed inland aquifer responses, including seawater intrusion dynamics. This integrated terrestrial–offshore perspective is particularly relevant for improving the assessment and management of coastal aquifers, including seawater intrusion and submarine groundwater discharge.

Aknowledgements: This research has been supported by the project MUCHOGUSTO (PID2022-140862OB-C21 and PID2022-140862OB-C22 funded by MCIN/AEI/10.13039/501100011033/ and “FEDER Una manera de hacer Europa”) anf SecuCoast financed for the European Commission and Spanish Research Council (AEI)  under the 2023 Joint call of the European Partnership 101060874 — Water4All.